I'm following this tutorial to validate a SSH server, in theory I should use a private key provided by an authoring server to sign the SSH pub key, but I don't understand the role of the -I <key_id> option and what value should it have:

1 Answer
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-I certificate_identity
Specify the key identity when signing a public key. Please see the CERTIFICATES section for details.
CERTIFICATES
ssh-keygen supports signing of keys to produce certificates that may be used for user or host authentication. Certificates consist of a public key, some
identity information, zero or more principal (user or host) names and a set of options that are signed by a Certification Authority (CA) key. Clients or
servers may then trust only the CA key and verify its signature on a certificate rather than trusting many user/host keys. Note that OpenSSH certifi‐
cates are a different, and much simpler, format to the X.509 certificates used in ssl(8).
ssh-keygen supports two types of certificates: user and host. User certificates authenticate users to servers, whereas host certificates authenticate
server hosts to users. To generate a user certificate:
$ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
The resultant certificate will be placed in /path/to/user_key-cert.pub. A host certificate requires the -h option:
$ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
The host certificate will be output to /path/to/host_key-cert.pub.
It is possible to sign using a CA key stored in a PKCS#11 token by providing the token library using -D and identifying the CA key by providing its pub‐
lic half as an argument to -s:
$ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
In all cases, key_id is a "key identifier" that is logged by the server when the certificate is used for authentication.
Certificates may be limited to be valid for a set of principal (user/host) names. By default, generated certificates are valid for all users or hosts.
To generate a certificate for a specified set of principals:
$ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
$ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub
Additional limitations on the validity and use of user certificates may be specified through certificate options. A certificate option may disable fea‐
tures of the SSH session, may be valid only when presented from particular source addresses or may force the use of a specific command. For a list of
valid certificate options, see the documentation for the -O option above.
Finally, certificates may be defined with a validity lifetime. The -V option allows specification of certificate start and end times. A certificate
that is presented at a time outside this range will not be considered valid. By default, certificates are valid from UNIX Epoch to the distant future.
For certificates to be used for user or host authentication, the CA public key must be trusted by sshd(8) or ssh(1). Please refer to those manual pages
for details.

Note:

In all cases, key_id is a "key identifier" that is logged by the server when the certificate is used for authentication.

And from your link it states:

-I: This is a name that is used to identify the certificate. It is used for logging purposes when the certificate is used for authentication.

When using certificates for authentication, these have keys to identify the the particular certificate of authentication. This is given any name you like I could call mine my-new-cert-id and the certificate will be built with that id and used to access my authentication certificate.

Note that in that link you provided two different -I names were used so choose that which you like.

Role of -I: ssh-keygen supports signing of keys to produce certificates that may be used for user or host authentication. Used when you need to use certificates for authentication.